(1) Field of the Invention
This invention relates to the domain of altimetric measurements and more particularly the determination of surface ocean currents using altimetric radar onboard satellites.
It is particularly but not exclusively applicable to the study of surface sea currents in coastal regions.
(2) Prior Art
Such a study is desirable for many reasons. Coastal waters are extremely dynamic regions in which submarine relief and particularly navigation channels can vary significantly over relatively short periods. Furthermore, the study of surface currents provides a means of determining the distribution of pollutants and the flow of cold polar water and water derived from ice melt.
Conventional satellite altimetric radar can scan a relatively narrow band also called a “swath” on the earth's surface, typically a few kilometres wide. This means that only a small fraction of the ocean surface can be observed during each orbital revolution, with a considerable space between two adjacent passes. At temperate latitudes, the space between passes is very much greater than the width of the scanned surface. The result is that it is necessary to make assumptions and approximations in order to deduce ocean circulation models from these observations and to solve turbulence problems.
INSAR (Interferometric Synthetic Aperture Radar) systems are also used to make altimetric measurements. Such a system comprises two antennas spaced at a fixed distance from each other, either parallel (ATI—Along-Track Interferometry) or perpendicular (XTI—Cross-Track Interferometry) to the track of the platform on which the system is onboard.
Along-Track Interferometry (ATI) systems are conventionally used to make current measurements. They are based on use of the fact that the phase of signals back-scattered from a moving target varies as a function of time at a rate that is determined by the speed of the sighting axis of the target (this effect corresponds to the Doppler frequency shift). This system is capable of acquiring two complex SAR (Synthetic Aperture Radar) images of a region with a short time offset, which shows up a phase shift proportional to the time offset and the rate of the phase shift. In this way, the measured phase shift can be converted into a target speed. For example, such a system is described in document [1] “Synthetic aperture radar interferometry applied to ship-generated waves in the 1989 Loch Linnhe Experiment”, D. R. Thompson & J. R. Jensen, J. Geophys. Res., 98, 10, 259-10, 269, 1993.
Cross-Track Interferometry (XTI) systems use two SAR antennas at a spacing from each other in a direction perpendicular to the track. In this case, the phase shifts depend on the topography of the illuminated area. Therefore, XTI systems are used essentially for the generation of land surface elevation models. They cannot be used for mapping the speed of mobile targets. However at sea, an XTI system can be used to measure sea level variations, and therefore to demonstrate current phenomena at medium scale. XTI systems have the advantage that they can scan wider bands with an equivalent precision and spatial resolution than conventional altimetry systems. Furthermore, XTI systems do not require that corrections due to atmospheric conditions are applied.
A Wide Swath Ocean Altimeter (WSOA) interferometer system was thus proposed capable of scanning a band about 200 km wide. This system is shown diagrammatically in FIG. 1 showing a top view of the antennas and the shape of the beams that they emit. As shown in this figure, this system comprises two interferometric antennas 1, 2 aiming at the nadir, fixed to the ends of an arm with length B called an “antenna base” arranged perpendicular to the track (along the y axis) of the satellite. Each antenna emits two beams 3, 4 and 5, 6 scanning two bands approximately 100 km wide, on each side of the trace of the satellite track on the ground. A map of the surface sea can be produced using Cross-Track Interferometry (XTI) to determine surface slopes providing useful information about ocean currents, so as to create a better model of the global temperature and salinity of the oceans and local phenomena.
However, neither conventional altimeters nor the WSOA system are capable of studying coastal regions due to their low precision and insufficient time sampling, and in any case are unsuitable for the large variability of these regions both in space and in time.
In “First Demonstration of Surface Currents Imaged by Hybrid Along—and Cross-Track Interferometry SAR ”, R. Siegmund et al. IEEE Transactions on Geoscience and Remote Sensing , Vol. 42, No. 3, pp. 511-519, March 2004, a system combining ATI Along-Track and XTI Cross-Track Interferometry techniques was envisaged to simultaneously obtain altitude and velocity measurements of surface currents. This system includes two antennas with an antenna base with a component along the line of the track and a component perpendicular to this line. This system can be used to obtain surface current velocities from combined interferometric phases and a geometric phase interpretation model only along a direction perpendicular to the track. Therefore, this system cannot be used to obtain a map of surface current velocities. Furthermore, the precision obtained using this system onboard an aircraft is of the order of 20 cm/s. If this system is onboard a satellite at an altitude of several hundred km, the precision obtained is unsuitable for the study of surface currents in coastal regions.
In “Study on Concepts for Radar Interferometry from Satellites for Ocean (and Land) Applications (Koriolis)”, R. Romeiser et al., DLR Study 50EE000, April 2002, it was also envisaged to combine the ATI and XTI techniques. This document shows that there is an optimum angle of attack of beams output from antennas equal to between 30° and 45°, to make surface current measurements with a relatively high precision. However, application of this constraint can degrade performances of the system concerning measurement of altitudes. It is also preferable to keep the scanned surface close to the nadir so that altimetric measurements made on a wide swath remain compatible with altimetric measurements made using a conventional altimeter pointing towards the nadir.